On the infinity pipe area: For reference, ignoring the wall thickness, dual 3.5" pipe gas area of 19.2 sqin. The infinity pipe with 6" overall width and 1/2" segment taken out from both pipes will have an area of 17.6 sqin. It's only a 8-9% area reduction after the gas has already cooled somewhat and after the pulses have already combined. My judgment is that it's never going to be a bottleneck in a system that is dual 3.5" upstream of the infinity pipe. And most importantly, it fits.

Unfortunately, these aren’t any smooth perforated inner pipe old-fashioned glass packs that I’m showing in this post. They are just two 3.5” straight pipes merging into a 3.5x6 infinity pipe. Revenge of the nerds in the sense of high school geometry working in real life:

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Yeah, that's the motivation. I like to come up with sticky names for things. Sometimes they work, sometimes they don't.

Beyond just descriptive names, I think this pipe configuration is pretty interesting for cross-overs and "termination boxes". The welds shouldn't pull out with pulsing, and the support rods don't cause much flow resistance. There's ample room for the pulses to combine, but the flow is fairly unrestricted. Feeding the pipe from two parallel pipes with 5" center to center distance requires just two gentle 10 degree turns within the real estate my car has, and should flow well. The shallow entry angles should make the inlet work not too differently from a merge collector, although for full suction you'd probably want more of a choke there at the merge. The divorce into two separate pipes should flow extremely well, too. It can be constructed from standard size round pipe sections with just a band saw and high-school geometry. It might be a good alternative for an x-pipe where the packaging considerations make a longer single pipe section desirable.

I wouldn't think so, but not like it's tested in any way at this point. In which way is it "anti-flow like"?

Flow from other half generates two tumbles to other which then also crash to each other. Basically flow in other pipe half pretty much stops flow in other half.

Could be, we'll see. My prediction is different. I'm predicting that we'll get a big drop in the downpipe back pressure compared to two 3" diameter separate pipes that fit in the same shape. The sensor is sufficiently upstream of this infinity pipe, so while not an independent single change comparison, it should be informative.

In terms of the tumbles you are predicting, are you talking of tumbles that come at the merge and rotate on the horizontal plane or tumbles that come after during the combined pipe section that rotate on the vertical plane?

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My prediction on those tumbles is that you'll get a bunch of them at low rpms but at full load high rpms the flow is going to stay mostly attached to the own side of the pipe. That's how it tends to work in x-pipes that are constructed the same way.

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I'm trying to play catch up here, the "infinity pipe" is a light bulb moment with, and at this time has little theory or testing to back it up, ie an experiment? I'm not against that, many ideas have been stumbled upon, just wondering how this one has been conceived.

I'm trying to play catch up here, the "infinity pipe" is a light bulb moment with, and at this time has little theory or testing to back it up, ie an experiment? I'm not against that, many ideas have been stumbled upon, just wondering how this one has been conceived.

What I need is something that fits in the 6"x"3.5 space envelope, has structural integrity, flows as much as possible, and combines pulses for sound purposes. I don't need pulse tuning to produce more power, because this is pretty far downstream of the turbines. It just needs to sound good and not cause much of a pressure loss at high loads and high rpms. It also needs to not drum at all, and have high structural integrity when subjected to the exhaust pulses. The welds can't fatigue and fail. John also fabricated an oval pipe for this section, but it's going to need a lot of internal support structures to not drum and not break the welds over time.

I've seen the same construction in exactly one exhaust system. This is the IPE F1 BMW M3 S65 exhaust. It has the reputation of sounding very good:

Here's a video what a cross-plane 90-degree V8 sounds like with that infinity pipe center section with pulses fully combined:https://youtu.be/SbT0IqFIfnc

Simple things like dual 3.5" into single round 5" and then back to dual 3.5" will not fit in this space envelope.

In any case, from flow perspective I'd expect this infinity pipe to work about the same way as an X-pipe that has almost no choke at the middle. You'd probably choke a NA X-pipe to about 75% of the combined inlet area, this pipe is closer to 91% of the combined inlet area because the point is not use pulses to generate more suction in one pipe at the expense of a temporarily little higher pressure in the other pipe.

I have been following this thread with interest. It's great to see a project that you guys obviously have a lot of time invested in coming to fruition, all layed out on a free forum with plenty of pictures

I have been following this thread with interest. It's great to see a project that you guys obviously have a lot of time invested in coming to fruition, all layed out on a free forum with plenty of pictures Thanks for sharing, and I can't wait to see / hear the results!!

Thanks for the encouragement. This project has been years in the works, for two reasons. First, both John and I have day jobs and many other unrelated projects, so this is worked on during the free time that is left over after other projects and family activities. Second, every time the car runs for a couple of weeks, one of us has taken it apart for no good reason than trying something new. It's basically a big science experiment for us, admittedly in an area where "the science is settled" to a large extent (higher density in the intake manifold does in fact make more power).